The present invention relates to pathogen resistant plants and in particular to pathogen resistant plants wherein pathogen resistance is triggered in response to invading pathogens such as viruses, DNA constructs for use in such plants and methods of introducing virus induced resistance into plants.
Viral infections in plants are frequently responsible for detrimental effects in growth, undesirable morphological changes, decreased yield and the like. Such infections often result in a higher susceptibility to infection in infected plants to other plant pathogens and plant pests.
Virus particles generally comprise a relatively small amount of genetic material (single or double stranded RNA or DNA) protected by a protein or proteins which in some viral types can also be surrounded with host-derived lipid membranes, yielding infectious particles. Viruses are dependent on host cells for multiplication and may therefore be regarded as intracellular parasites.
Plants have evolved a number of defensive mechanisms to limit the effects of viral infection. For example, so-called horizontal or partial resistances which are polygenic in nature and so-called vertical resistances which are monogenic in nature.
Horizontal resistance is difficult to introduce successfully into plants in breeding programs, however, vertical resistance can be bred into plants relatively easily within plant breeding programs. Genes coding for virus resistance can act constitutively in a passive sense, ie without a requirement for inducing gene expression. Constitutively expressed virus resistances include as modes of action non-host resistances, tolerance ie inhibition of disease establishment, immunity ie inhibition of transport or the presence of antiviral agents and the like. Alternatively, genes coding for virus resistance in plants can be actively switched on by way of inducing expression of a gene or genes encoding for a viral resistance. An example of such a system includes the hypersensitive response.
So-called hypersensitive responses (HSR) in plants have been reported and are generally characterized by death of plant cells in the vicinity of the penetrating pathogen shortly after infection. Movement of the pathogen through infected or invaded cells is restricted or blocked due to necrosis of the invaded cell and/or cells in the environs of the invaded cell(s). In addition, HSR involves a cascade of additional or secondary defense responses and the accumulation of certain proteins and secondary metabolites, leading to a general increased level of resistance to attack by pathogens. HSR reactions to invading organisms are generally thought to involve a resistance gene product in the plant cell which recognizes and interacts with an elicitor element, ie the product of an avirulence gene of a pathogen. Elicitor element recognition in the cells of a resistant plant triggers an HSR reaction which in its turn restricts the pathogen infection to a single cell or cells, or at most to a few plant cells in the immediate vicinity thereof.
An example of HSR-mediated resistance to virus infection is that of tobacco plants harbouring the N' resistance gene to tobamoviruses such as TMV and ToMV, which contain the coat protein avirulence gene. Thus far, more than twenty single dominant HSR-type resistance genes have been identified, and are present in many agronomically important crops including tobacco, tomato, potato, pepper, lettuce, and the like.
Despite the apparent abundance of resistance sources to certain viruses, many crops still lack effective resistance genes to important viral pathogens Fraser, R. S. S. (1992). Euphytica 63:175!. Searching of wild type germplasm collections has identified only a few suitable sources of viral resistance capable of being introduced successfully into agronomically important crops. An example is the absence of vertical resistance genes to cucumber mosaic virus (CMV) in many agronomically important crop types including but not limited to tomato, pepper, cucumber, melon, lettuce and the like.
Plant breeders continuously try to develop varieties of crop plant species tolerant to or resistant to specific virus strains. In the past, virus resistance conferring genes have been transferred from wild types related to commercial plants into commercial varieties through breeding. The transfer of an existing resistance in the wild from the wild type gene pool to a cultivar is a tedious process in which the resistance conferring gene(s) must first be identified in a source (donor) plant species and then combined into the gene pool of a commercial variety. Resistance or tolerance generated in this way is typically active only against one or at best a few strains of the virus in question. A further disadvantage is that the breeding programme generally takes a long time, measured in years, in getting to agronomically useful plants.
In an alternative, a system referred to as "cross-protection" has been employed. Cross-protection is a phenomenon in which infection of a plant with one strain of a virus protects that plant against superinfection with a second related virus strain. The cross-protection method preferentially involves the use of avirulent virus strains to infect plants, which act to inhibit a secondary infection with a virulent strain of the same virus. However, the use of a natural cross-protection system can have several disadvantages. The method is very labour intensive because it requires inoculation of each individual plant crop, and carries the risk that an avirulent strain may mutate to a virulent strain, thus becoming a causal agent for crop disease in itself. A further possible hazard is that an avirulent virus strain in one plant species can act as a virulent strain in another plant species.
Genetically engineered cross-protection is a form of virus resistance which phenotypically resembles natural cross-protection, but is achieved through the expression of genetic information of a viral coat protein from the genome of a genetically manipulated plant. It is known that expression of the tobacco mosaic virus strain U1 (TMV-U1) coat protein gene from the genome of a transgenic plant can result in a delay of symptom development after infection with any TMV strain. Similarly, coat protein-mediated protection has also been obtained for alfalfa mosaic virus (AMV), potato virus X (PVX) and cucumber mosaic virus (CMV). For some plant viruses, eg luteoviruses, it is difficult to obtain detectable amounts of the corresponding coat protein in a transgenic plant, and consequently, virus resistance is generally lowered. Furthermore, any alleged degree of protection requires that the plant produces coat protein continually and thus imposes an energy burden on the plant. As a result of such limitations the commercial value of such technology remains unclear.
A further example of genetically engineered virus resistance includes the introduction of plant viral satellite RNA wherein expression of incorporated genetic material modifies the plant virus or its effects.
An object of the present invention is to provide an alternative more reliable engineered virus resistance strategy in plants to those engineered resistances known in the art, based on direct pathogen induced expression of molecules in target tissues of a plant before the invading pathogen can establish itself in the host plant.
Another object of the invention is to combine genetic engineering plant transformation technology with naturally existing plant viral defense mechanisms in plant tissue.